Anatase titanium dioxide which is usually used as a photocatalyst removing environmental pollutants, as pigment material, additives for plastic product or for optical multi-coating reagent. Titanium dioxide has mainly three polymorphic forms of crystalline structure, that is anatase, brookite and rutile. The performance of titanium dioxide in various applications depends on its crystalline phase state, dimensions and morphology. Titanium dioxide with anatase phase has been used as a photocatalyst in different organic reactions. It is also used in photovolatics because of its high photoactivity. Titanium dioxide shows different electrical characteristics according to the oxygen partial pressure since it has wide chemical stability and non-stoichiometric phase region. Because of this it can also be used as a humidity sensor.
Impact of nanostructure on the properties of high surface area materials is an area of increasing importance for understanding, creating and improving materials for diverse applications. The synthesis of nanoparticles with controlled size and composition is of technological interest. Reference may be made to the chloride process commercialized by Du Pont in USA in 1956 for preparation of titanium dioxide powders by the chloride process wherein titanium tetrachloride vigorously reacted with air undergoing hydrolysis at 1000° C. The inherent disadvantage of the process is the use of high temperature and costly equipments to control the reactant mixing ratios and less control on the particle shape and size. Reference may be made to the sulfate process which was industrialized Titan company in Norway in 1916 wherein titanium sulfate is conventionally hydrolyzed at temperatures higher than 95° C. The disadvantage is the post calcination at 800-1000° C. to obtain the titanium dioxide powder.
The other chemical methods to obtain titanium dioxide powders include (a) hydrolysis with ammonium hydroxide solution (b) sol-gel method (c) hydrothermal synthesis (d) hydrodynamic cavitation etc.
The hydrolysis method suffers from the limitation that it necessitates a post-calcination of the precipitates of hydroxides to obtain the respective oxides. Reference may be made to U.S. Pat. No. 5,030,439 wherein a method is described to prepare particulate anatase titanium dioxide by reacting titanium tetrahalide with sulfuric acid at 65-100° C. to first form titanyl sulfate which is subsequently crystallized then re-dissolved in water and hydrolyzed at 85-100° C. to form titanium dioxide. The inherent disadvantage is that it is a two step process requiring subsequent heat treatment.
The conventional sol-gel method involves metal alkoxides which requires tight control of reaction conditions since alkoxides are intensely hydrolyzed in air.
Furthermore, the high price of alkoxides limits the commercialization of this process. Reference may be made to JP 9-124, 320 wherein the gel was formed by adding water to titanium tetrachloride dissolved in alcohol together with various kinds of acetates, oxalates and citrates containing alkali metals or alkaline earth metals. The inherent disadvantage is the use of expensive additives such as organic acids and needs a high temperature treatment after gel formation.
The hydrothermal synthesis needs high temperature and pressure conditions and hence requires the use of an autoclave. All the above wet chemical routes however involve a heat treatment either during the processing or as a post-calcination step. Reference may be made to the work of Bruno (U.S. Pat. No. 5,973,175, 1999) wherein titanium dioxide is prepared from amino titanium oxalate precursor by hydrothermal process. Reference may be made to U.S. Pat. No. 4,954,476 wherein a method to prepare a catalyst containing titanium dioxide as a primary ingredient in a hydrothermal process with meta- or ortho-titanic acid as starting material has been described. The inherent disadvantage of all the processes is the high temperature and pressure requirement. Reference may also be made to U.S. Pat. No. 3,242,557 wherein a process is described to prepare pigmentary titanium dioxide by hydrothermal precipitation. The inherent disadvantage is that during the reaction, the reaction mixture is subjected to ultrasonic vibrations.
Attempts to synthesize nanoparticles of oxides in particular include the above said chemical routes. But the inherent disadvantages are in controlling the agglomeration and particle growth, which is mainly caused because of the involved heat treatment. Attempts to use hydrazine hydrate are concentrated in obtaining metal nanoparticles like Silver where hydrazine hydrate is used as a strong reducing agent.
Reference may be made to the work of Pileni et al J. Phys. Chem. 1993, 97, 12974, wherein silver nanoparticles were prepared by reducing silver sulfosuccinate solution by hydrazine hydrate.
Hydrazine monohydrate has been used earlier to synthesize oxides like ferrites where hydrazine is used to form an intermediate which decomposes by self-ignition or self propagating high temperature synthesis to obtain the ultra fine powders of ferrites. In this context, reference may be made to the publications of Ravindranathan et. al J. Mat. Sci., 1986, 5, 221, wherein ν-ferric oxide was prepared by thermal decomposition of hydrazine precursors in air around 200° C. Also reference may be made to the work by Suresh et. al. J. Thermal Anal., 1989, 35, 1137 wherein Magnesium ferrite has been prepared by the thermal decomposition of a metal oxalate hydrazinate precursor. Reference may also be made to the work of Madhusudan Reddy et al., (J. Solid Slate Chem., 2001, 158, 180 & Mater. Chem. Phys., 2002, 78, 239) wherein hydrazine monohydrate is used with titanium tetrachloride to obtain anatase titanium dioxide nanoparticles with 5-15 nm. The inherent disadvantage is that the precipitate was air dried at 80-100° C. followed by heat treatment at 300-400° C.
The main difference in the procedure adopted by Madhusudan Reddy et al. (J. Solid State Chem., 2001, 158, 180 and Mater. Chem Phys., 2002, 78, 239) and the present invention for the synthesis of Nanoparticles of anatase titanium dioxide are the following.    1. Madhusudan Reddy et al obtained the crystalline Nanoparticles only after air drying the samples at 80-100° C. and then calcining at 300-400° C.    2. The Applicants' present conditions of temperature i.e., 20-40° C. and pressure around 1 atmosphere, and carrying out the reaction under nitrogen atmosphere; all of them result in Nanoparticles of anatase titanium dioxide unambiguously less than 5 nm; crystalline in nature with no heat treatment and calcinations.            Hydrazine monohydrochloride is a salt, which is obtained dissolved in the reaction medium, i.e., water. Freeze drying of the solution gives the salt in powder form. It is a deliquescent material hence the particle size cannot be obtained. However from XRD, the crystallite size can be estimated to be in the range of 15-20 nm.        
The present invention discloses the preparation of nanocrystalline anatase titanium dioxide powder of particle size less than 5 nm reacting acidic aqueous titanium tetrachloride solution with hydrazine monohydrate at ambient reaction conditions in a single step without any subsequent heat treatment.